place function IDs in correct scope

src/Module.js

@@ -535,7 +535,7 @@ export default class Module {
 			statement.references.forEach( reference => {
 				const declaration = reference.declaration;
 
-				if ( reference.declaration ) {
+				if ( declaration ) {
 					const { start, end } = reference;
 					const name = declaration.render( es6 );
 

src/Statement.js

@@ -135,7 +135,13 @@ export default class Statement {
 				}
 
 				if ( isReference( node, parent ) ) {
-					const reference = new Reference( node, scope );
+					// function declaration IDs are a special case – they're associated
+					// with the parent scope
+					const referenceScope = parent.type === 'FunctionDeclaration' && node === parent.id ?
+						scope.parent :
+						scope;
+
+					const reference = new Reference( node, referenceScope );
 					references.push( reference );
 
 					reference.isImmediatelyUsed = !readDepth;

src/ast/Scope.js

@@ -86,6 +86,7 @@ export default class Scope {
 			this.parent.addDeclaration( node, isBlockDeclaration, isVar );
 		} else {
 			extractNames( node.id ).forEach( name => {
+				if ( this.declarations[ name ] ) throw new Error( `Duplicate declaration (${name})` );
 				this.declarations[ name ] = new Declaration( name );
 			});
 		}

src/ast/attachScopes.js

@@ -11,8 +11,6 @@ export default function attachScopes ( statement ) {
 
 	walk( node, {
 		enter ( node, parent ) {
-			let newScope;
-
 			// function foo () {...}
 			// class Foo {...}
 			if ( /(Function|Class)Declaration/.test( node.type ) ) {
@@ -29,6 +27,8 @@ export default function attachScopes ( statement ) {
 				});
 			}
 
+			let newScope;
+
 			// create new function scope
 			if ( /Function/.test( node.type ) ) {
 				newScope = new Scope({